1. Field of the Invention
The present invention relates to an active noise control system for canceling out unwanted noise in a vehicle's passenger compartment with secondary noise produced out of phase with the unwanted noise (having 180°-shifted phase with respect to the phase of the unwanted noise in the passenger compartment), and more particularly to an active noise control system for canceling out unwanted noise in a closed space such as a vehicle's passenger compartment based on low-frequency road noise (also referred to as “drumming noise”) in frequencies ranging from 20 to 150 Hz which is produced due to the acoustic resonant characteristics of the closed space.
2. Description of the Related Art
Active noise control systems for attenuating drumming noise in a vehicle's passenger compartment have been based on feed-forward active control as shown in FIG. 26 of the accompanying drawings.
In FIG. 26, the conventional active noise control system has attenuated noise in a vehicle's passenger compartment 24 as follows: Suspension vibrations and vehicle body vibrations that are highly correlated to the noise in the passenger compartment are detected by sensors, and detected signals from the sensors are used as a reference signal. The reference signal is supplied to an adaptive digital filter 21 and a digital filter 22 whose transfer characteristics are made equivalent to the sound field transfer characteristics of the passenger compartment. The digital filter 22 supplies its output signal to a filter coefficient updating circuit 23 which calculates filter coefficients w1, w2, w3, . . . , wi of the adaptive digital filter 21 based on an LMS algorithm. The filter coefficients w1, w2, w3, . . . , wi calculated by the filter coefficient updating circuit 23 are set in the adaptive digital filter 21, which applies an output signal to drive a speaker 25 that functions as a secondary noise source placed in the passenger compartment 24 which serves as a sound field. The difference between a sound radiation outputted from the speaker 25 and noise in the passenger compartment 24 is detected by a microphone 26 which is provided in the passenger compartment 24 for confirming noise cancellation. An output signal from the microphone 26 is sent as an error signal to the filter coefficient updating circuit 23, which calculates filter coefficients w1, w2, w3, . . . , wi in order to eliminate the square of the error signal.
The adaptive digital filter 21, the digital filter 22, and the filter coefficient updating circuit 23 jointly make up a control means for being supplied with a signal highly correlated to a sound from a noise source as a reference signal and generating a noise canceling signal which is exactly out of phase to the noise in the passenger compartment 24. The speaker 25 serves as a canceling sound generating means for generating a noise canceling sound in response to the noise canceling signal outputted from the control means.
When the speaker 25 generates and radiates the noise canceling sound as secondary noise, the radiated secondary noise cancels out the noise in the passenger compartment 24 for thereby suppressing the noise in the passenger compartment 24.
Efforts have also been made to adjust the weight of a certain region of the vehicle body for attenuating noise generated in the passenger compartment by drumming noise.
With the conventional active noise control system, it is necessary to use a microphone for confirming noise cancellation in the passenger compartment and also a reference signal that is highly correlated to the noise in the passenger compartment and satisfies the causality.
For suppressing the noise in the passenger compartment based on low-frequency road noise, it is also necessary to obtain a reference signal that is highly correlated to the noise in the passenger compartment and satisfies the causality. However, it is very difficult to produce such a reference signal.
The difficulty arises out of the fact that the noise in the passenger compartment based on low-frequency road noise is affected more greatly by the acoustic resonant characteristics of the sound field in the vehicle body than by vibrational characteristics of the suspensions and various vehicle body regions.
Japanese laid-open patent publication No. 5-273987 discloses a conventional active noise control system having a microphone for confirming noise cancellation which is mounted on a side of the headrest of a front seat in a passenger compartment. The microphone detects noise in the vicinity of the ears of a passenger seated on the front seat to cancel noise in the passenger compartment. In the disclosed conventional active noise control system, the understanding of transfer characteristics with respect to sounds in the passenger compartment, particularly transfer functions between a speaker as a secondary noise source and the microphone for confirming noise cancellation, has an important effect on the noise cancellation capability of the system.
However, since the front seat with the microphone mounted on the side of the headrest thereof is adjustable in position, when the front seat is moved forward or backward, the position of the microphone is changed in the passenger compartment. When the microphone is changed in position, the relative position between the speaker and the microphone is also changed. As a result, the transfer function between the speaker and the microphone is varied, and the noise in the passenger compartment cannot sufficiently be attenuated.
The above problem also occurs when the angle of the backrest of the front seat is changed.
The approach to adjust the weight of a certain region of the vehicle body for attenuating drumming noise is disadvantageous in that it has to rely upon a process of trial and error and hence is tedious and time-consuming, and the attempt usually brings about an increase in the weight of the vehicle body region.